No way to pin model version (claude-opus-4-5 vs 4-6)

Problem

There's no way to pin a specific model version in Claude Code. When Anthropic deploys a new version (e.g. 4.5 → 4.6), all users are forced onto it with no rollback option.

--model accepts family names (opus, sonnet) but not pinned version IDs like claude-opus-4-5-20250918.

Why this matters

Model version upgrades can introduce regressions in instruction-following. In my case, 4.6 consistently:

• Writes code against APIs it imagines exist without reading the actual source
• Claims work is complete when it isn't
• Reads project instructions (CLAUDE.md) then immediately violates them
• Updates documentation to match broken code instead of fixing the code

These behaviors were less frequent on 4.5. With no version pinning, there's no way to stay on a working version while regressions get sorted out.

Requested

• Allow --model claude-opus-4-5-20250918 (or similar pinned IDs) in Claude Code
• Or add a config option to pin model version in .claude/settings.json

Customers shouldn't be involuntary QA for model upgrades on production codebases.